I am 5 months into my last Master-Cleanse and planning to start again next Monday. During most of this time, I ate and drank much healthier than before I did pre-MC. Although, I have to admit I am back on Coke, I was caffeine free for 5 months. The reason I am going back to the Master-Cleanse is not because I cheated. I am doing the Master-Cleanse again because no matter how healthy we eat our bodies still absorb so much crap (for a lack of a better term). I do the MC for a healthier me.
I think the most important thing that the MC has to teach you, is you have self control over what you put in your body. You will have to have control to say no or reward yourself but in moderation at times. Also, you will need to learn how to reward yourself in other ways besides food.